How do aerosols from volcanic eruptions effect climate?

Aerosols (sulfates) from volcanic eruptions can actually cool the earth's average temperature by blocking incoming solar radiation. This has happened before several times after catastrophic eruptions. Aerosols can also be released from some algae in the ocean (coccolithophorids) in the form of dimethylsulfate, which forms sulfuric acid particles when it reacts with water in the atmosphere. This will have the same effect as aerosols from volcanoes. The big question is: can aerosols slow down global warming? That depends on how much is in the atmosphere compared to the increasing concentration of carbon dioxide.